X-ray image sensor and method for fabricating the same

ABSTRACT

An X-ray image sensor having a photoelectric conversion part that converts X-ray photons into electric charges. The X-ray image sensor includes a pixel electrode for collecting the electric charges and a storage capacitor for storing the electric charges collected by the pixel electrode. The storage capacitor includes a first capacitor electrode, the pixel electrode, and a dielectric layer that is deposited on the first capacitor electrode. The pixel electrode contacts an electron transport electrode via a hole through the dielectric layer. A switching TFT controls the release of electric charges in the storage capacitor to an external circuit. The switching TFT is comprised of a gate electrode, a first insulation film, a drain electrode, and a source electrode that contacts the electron transport electrode.

This application is a Division of application Ser. No. 09/722,654 Filedon Nov. 28, 2000 U.S. Pat. No. 6,399,962.

This application claims the benefit of Korean Patent Application No.1999-53712, filed on Nov. 30, 2000, which is hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to X-ray image sensors. More particularly,it relates to X-ray image sensors having a TFT (Thin Film Transistor)array, and to a method for fabricating the same.

2. Discussion of the Related Art

X-ray detection has been widely used for medical diagnosis. X-raydetection typically uses an X-ray film to produce a photograph.Therefore, some predetermined developing and printing procedures arerequired to produce the photograph.

However, digital X-ray image sensors that employ TFTs (Thin FilmTransistors) have been developed. Such X-ray image sensors have theadvantage that real time diagnosis can be obtained.

FIG. 1 is a schematic, cross-sectional view illustrating the structureand operation of an X-ray image sensing device 100. Included are a lowersubstrate 1, a thin film transistor 3, a storage capacitor 10, a pixelelectrode 12, a photoconductive film 2, a protection film 20, aconductive electrode 24 and a high voltage D.C. (direct current) powersupply 26.

The photoconductive film 2 produces electron-hole pairs in proportion tothe strength of external signals (such as incident electromagnetic wavesor magnetic waves). That is, the photoconductive film 2 acts as aconverter that converts external signals, particularly X-rays, intoelectric signals. Either the electrons or the holes are then gathered bythe pixel electrode 12 as charges. The pixel electrode is locatedbeneath the photoconductive film 2. Which charge species that isgathered depends on the voltage (Ev) polarity that is applied to theconductive electrode 24 by the high voltage D.C. power supply 26. Thegathered species charges are accumulated in the storage capacitor 10,which is formed in connection with a grounding line. Charges in thestorage capacitor 10 are then selectively transferred through the TFT 3,which is controlled externally, to an external image display device thatforms an X-ray image.

In such an X-ray image sensing device, to detect and convert weak X-raysignals into electric charges it is beneficial to decrease the trapstate density (for the electric charge) in the photoconductive film 2,and to decrease charge flow in non-vertical directions. Decreasingnon-vertical charge flow is usually accomplished by applying arelatively high voltage between the conductive electrode 24 and thepixel electrode 12.

Electric charges in the photoconductive film 2 are trapped and gatherednot only on the pixel electrode 12, but also over the channel region ofthe TFT 3. Even during the OFF state, the electric charges trapped andgathered on the pixel electrode 12 and on the channel region of the TFT3 induce a potential difference between the TFT 3 and the pixelelectrode. This has a similar effect as the TFT 3 being in the ON state.This adversely affects the switching of the TFT 3 and increases the OFFstate leakage current. Such can result in an undesired image.

FIG. 2 is a plan view illustrating one pixel 102 of the X-ray imagesensor panel 100. Shown are the TFT 3, a storage capacitor “S” and apixel electrode 62 that collects charges.

The TFT 3 includes a gate electrode 31, which is formed by an elongationof a gate line 50, and a drain electrode 32, which is formed by anelongation of a drain line 52.

The storage capacitor “S” is comprised of transparent first and secondcapacitor electrodes 58 and 60. A ground line 42 acts as a commonelectrode that is shared by adjacent pixels. Also shown are firstcontact holes 54 that connects the pixel electrode 62 with a sourceelectrode 33 of the TFT 3, and a second contact hole 56 that connectsthe pixel electrode 62 with the second capacitor electrode 60.

According to the conventional art, an X-ray image sensor includes aphotoelectric conversion part that produces electric charges inaccordance with received electromagnetic energy; a charge storagecapacitor “S” having a first capacitor electrode 58, a dielectric layerthat is deposited on the first capacitor electrode 58, a secondcapacitor electrode 60 on the dielectric layer, a protection film havingmultiple contact hole(s) 54 and 56 on the second capacitor electrode 60,and a pixel electrode 62 that is formed on the protection film. Thepixel electrode is in contact with the second capacitor electrode 60through the contact hole(s) 56 and collects the electric chargesproduced in the photoelectric conversion part. A switching TFT 3controls the release of the electric charges stored in the storagecapacitor “S”. The switching TFT includes a gate electrode 31, a drainelectrode 32, and a source electrode 33 that contacts the pixelelectrode 62.

FIGS. 3a to 3 f are sectional views, taken along the line III—III ofFIG. 2, that illustrate a manufacturing process.

Referring to FIG. 3a, a metal layer is deposited and patterned on asubstrate 1 to form a taper-shaped gate electrode 31. The substrate 1can be a quartz substrate or a glass substrate. However, the substrate 1is beneficially a glass panel since quartz panels are relativelyexpensive. The gate electrode 31 can made of a metallic materialselected from a group comprised of Molybdenum (Mo), Tantalum (Ta),Tungsten (W), Niobium (Nb), and Antimony (Sb).

FIG. 3b illustrates the steps of depositing a first insulation film 102and a semiconductor layer 104. The gate insulation film 102 is formed bya deposition of an inorganic insulation film (such as a silicon nitride(SiN_(x)) film or a silicon oxide(SiO_(x)) film) having 4000 Åthickness. Alternatively, an organic insulation material such as BCB(benzocyclobutene) or acrylic resin can be used. After the deposition ofthe first insulation film 102, a dual layer semiconductor film 104comprised of an amorphous silicon layer 104 a and a doped amorphoussilicon film 104 b are deposited. Although vapor deposition or ioninjection can be used for the formation of the doped amorphous siliconfilm 104 b, vapor deposition is usually employed.

Next, as shown FIG. 3c, a second metal layer is deposited for both thesource electrode 33 and the drain electrode 32, and for the sound line42. That metal, beneficially Chromium (Cr) or a Cr-alloy, is thenpatterned to form the source electrode 33, the drain electrode 32 andthe ground line 42. Moreover, the portion of the doped amorphous siliconfilm 104 b between the source and drain electrodes 33 and 32 iseliminated by using the source and drain electrodes as masks. Then, afirst capacitor electrode 58 is formed over the ground line 42. Thefirst capacitor electrode 58 is beneficially comprised of a transparentelectrode material such as ITO (indium tin oxide). The region C in FIG.3c designates a switching transistor.

Referring to FIG. 3d, a silicon nitride film having a thickness of 3000Å forms a second insulation film 106 is deposited on the source anddrain electrodes 33 and 32, and on the first capacitor electrode 58. Thesecond insulating film 106 acts as protective layer for the TFT 3 and asa dielectric for a capacitor that is being formed with the firstcapacitor electrode 58.

After the second insulation film 106 is deposited a second capacitorelectrode 60 is formed on the second insulation film 105 and over thefirst capacitor electrode 58. Beneficially, the second capacitorelectrode is the same size as or a little larger than the firstcapacitor electrode 58.

As shown in FIG. 3e, an insulating protection film 108 is then formed.An organic substance such as BCB (benzocyclobutene) is beneficiallyused. BCB is a material that has a lower dielectric constant thansilicon nitride, silicon oxide or acrylic resin. After formation of theprotection film 108, first and second contact holes 54 and 56 are formedthrough the protection film 108. The first contact hole 54 exposes aportion of the source electrode 33. The second contact hole 56 exposes aportion of the second capacitor electrode 60. Although the first contacthole 54 penetrates down to the source electrode 33, the second contacthole 56 can not go as deep since the second capacitor electrode 60 actsas an etch stop that prevents the second insulation film 106 from beingetched.

FIG. 3f illustrates the step of forming a pixel electrode 62 (a thirdtransparent electrode layer). The pixel electrode is formed over thesecond insulation film 106 such that the pixel electrode extends intothe first and second contact holes 54 and 56 and electrically connectswith the source electrode and the second capacitor electrode 60. Inaddition, the pixel electrode 62 is formed such that it extends over theTFT 3.

The next step is the application of a light-sensitive material 123. Thatmaterial converts received external signals (X-rays) into electriccharges. The light-sensitive material 123 is beneficially comprised ofan amorphous selenium compound that is deposited in a thickness of 100to 500 μm by an evaporator. However, other X-ray-sensitive materialsthat having low dark conductivity and high sensitivity to externalsignals, for example HgI₂, PbO₂, CdTe, CdSe Thallium bromide or cadmiumsulfide can also be used. When the light-sensitive material is exposedto X-rays, electron-hole pairs are produced in the light-sensitivematerial in accordance with the strength of the x-rays.

After the application of the X-ray-sensitive material, a transparentconductive electrode 133 that passes X-ray is formed. When a voltage isapplied to the transparent conductive electrode 133 while X-rays arcbeing irradiated electron-hole pairs formed in the light-sensitivematerial are separated into charges that are gathered to the pixelelectrode 62 and stored in the storage capacitor “S”.

According to the mentioned conventional X-ray image sensing device,however, the depositing and patterning of the electrodes are performedthree times to fabricating the storage capacitor “S”. Moreover, the gateline, and the source and drain electrode are overetched while etchingthe ITO layers.

Moreover, as shown FIG. 4, due to the shortness of the length “ΔL”between the first capacitor electrode 58 and the source electrode 33, ashort-circuit can result.

Furthermore, a parasitic capacitor between the drain line 52 and thefirst capacitor electrode 58 can cause problems.

The present invention has been developed as a result of continuouseffort by the inventors to solve the above-described problems.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an x-ray image sensorand to a method for fabricating the same and that substantially obviatesone or more of the problems due to limitations and disadvantages of therelated art.

An object of the present invention is to provide an X-ray image sensorhaving simpler processing steps while forming ITO (indium tin oxide)electrodes.

Another object of the present invention is to provide an X-ray imagesensor having improved yields.

A further object of the invention is to provide a method of forming anX-ray image sensor which can reduce processing error during productionby preventing short-circuits and which can decrease noise due to aparasitic capacitor.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve the above objects, the present invention provides an X-rayimage sensor, including: a substrate; a gate electrode on the substrate;a first insulation fill on the substrate that covers the gate electrode;a semiconductor film of amorphous silicon film on the first insulationfilm over the gate electrode; a doped amorphous silicon film on thesemiconductor film; source and drain electrodes on the doped amorphoussilicon film that are spaced apart from each other; a ground line on thefirst insulation film that is spaced apart from the source and gateelectrodes; a second insulation film covering the whole substrate andhaving first and second contact holes that expose portions of the groundline and the source electrode, respectively; a first capacitor electrodeon the second insulation film, the first capacitor electrode having anelectrical connection with the ground line through the first contacthole; an electron transport electrode on the second insulation film, theelectron transport electrode having an electrical connection with thesource electrode through the second contact hole; a dielectric layercovering the second insulation film, the first capacitor electrode andthe electron transport electrode, the dielectric layer having a thirdcontact hole; and a pixel electrode on the dielectric layer having anelectrical connection with the electron transport electrode.

Beneficially, the pixel electrode extends over the semiconductor film,and the insulation layers are made of a material selected from the groupconsisting of BCB (benzocyclobutene), acryl and polyamide. Moreover, theX-ray image sensor includes a light-sensitive material on the pixelelectrode.

In order to achieve the above objects, the invention also provides amethod for fabricating an X-ray image sensor including: providing asubstrate; forming a gate electrode on the substrate; forming a firstinsulation film on the substrate and that covers the gate electrode;forming a semiconductor film of amorphous silicon film on the firstinsulation film over the gate electrode; forming a doped amorphoussilicon film on the semiconductor film; forming source and drainelectrodes on the doped amorphous silicon film that are spaced apartfrom each other, forming a channel region by eliminating the dopedamorphous silicon film between the source and drain electrodes by usingthe source and drain electrodes as a mask; forming a ground line on thefirst insulation film that is spaced apart from the source and gateelectrodes; forming a second insulation film covering the wholesubstrate; forming first and second contact holes that expose a portionof the ground line and a portion of the source electrode, respectively;forming a first capacitor electrode on the second insulation film, thefirst capacitor electrode having an electrical connection with theground line through the first contact hole; forming an electrontransport electrode on the second insulation film, the electrontransport electrode having an electrical connection with the sourceelectrode through the second contact hole; forming a dielectric layerthat covers the second insulation film, the first capacitor electrodeand the electron transport electrode; forming a third contact hole inthe dielectric layer; forming a pixel electron on the dielectric layer,the pixel electrode having an electrical connection with the electrontransport electrode; forming a light-sensitive material on the pixelelectrode; and forming a transparent conductive electrode that passesX-rays on the light-sensitive material.

The insulation layers are beneficially made of a material selected fromthe group consisting of BCB (benzocyclobutene), acryl and polyamide. Thecapacitor electrode and the pixel electrode are beneficially made oftransparent ITO (indium tin oxide). The X-ray-sensitive material isbeneficially one of a group consisting of HgI₂, PbO₂, CdTe, CdSe,Thallium Bromide, and Cadmium Sulfide etc.

It is to be understood that both toe foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is a cross-sectional view illustrating the principle of operationof an X-ray image sensor;

FIG. 2 is a plan view illustrating one pixel of an X-ray image sensoraccording to a conventional art;

FIGS. 3a to 3 f are processing diagrams corresponding to a cross-sectionof FIG. 2 (III—III) and sequentially illustrate the manufacture of anX-ray image sensor according to the conventional art;

FIG. 4 is an enlarged cross-sectional view illustrating a portion “G” ofFIG. 3f; and

FIGS. 5a to 5 f are processing diagrams illustrating the manufacture ofan X-ray image sensor according to the present invention.

DETAILED DESCRIPTION OF AN ILLUSTRATED EMBODIMENT

Reference will now be made in detail to an illustrated embodiment of thepresent invention, the example of which is shown in the accompanyingdrawings.

According to the present invention, an X-ray image sensor comprises aphotoelectric conversion part that produces electric charges inaccordance with the received amount of light; a charge storage capacitorhaving a second insulation film with a plurality of contact holes for afirst capacitor electrode, the first capacitor electrode and an electrontransport electrode on the second insulation film, a dielectric layerdeposited on the first capacitor and electron transport electrodes, anda pixel electrode formed on the dielectric layer in contact with theelectron transport electrode through a contact hole, the electrontransport electrode for collecting the electric charges produced in thephotoelectric conversion part; and a switching TFT that controls therelease of electric charges stored in the storage capacitor.

The switching TFT is comprised of a gate electrode, a first insulationfilm, a drain electrode, and a source electrode that contacts theelectron transport electrode.

FIGS. 5a to 5 e are diagrams that help illustrate the process ofmanufacturing an X-ray image sensor according to the principles of thepresent invention. Such an X-ray image sensor uses only two electrodesfor a storage capacitor.

Referring to FIG. 5a, an opaque first metal layer is deposited on asubstrate 1. The first metal layer is then patterned to form a gateelectrode 200.

FIG. 5b is a cross-sectional view illustrating the depositing of a firstinsulation film 202 and of a semiconductor layer 208. The (gateinsulating) first insulation film 202 is formed over the substrate bydeposition. Then, a dual layer semiconductor film 208 comprised of apure amorphous silicon film 204 and of a doped amorphous silicon film206 is deposited and patterned. The semiconductor film 203 can also bemade of polycrystalline silicon.

As shown in FIG. 5c, the next step is the formation of a second metallayer for source and drain electrodes 212 and 210, and for a ground line214. The second metal layer is deposited on the semiconductor layer 208and on the first insulation layer 202. Then, the source and drainelectrodes 212 and 210 are formed such that they are spaced apart bypatterning the second metal layer on the semiconductor layer 208. Theground line 214 is also formed on the first insulation film 202, againby patterning the second metal layer.

After that, a portion of the doped amorphous silicon film 206 betweenthe source an drain electrodes 212 and 210 is eliminated by using thesource and drain electrodes 212 and 210 as masks. This defines a channelregion “Ch”. The thin film transistor (TFT) “T” is this completed. Inoperation the ground line 214 removes residual charge in a storagecapacitor that is being formed.

FIG. 5d is a cross-sectional view illustrating the fabrication step of afirst capacitor electrode 222 and of an electron transport electrode224.

A second insulation film 216 that acts as a protection film is formedover the TFT an the ground line 214. After the formation of the secondinsulation film 216, first and second contact holes 220 and 218 areformed through the second insulation film. The first contact hole 220exposes a portion of the ground line 214, while the second contact hole218 exposes a portion of the source electrode 212.

Then, a first capacitor electrode 222 and an electron transportelectrode 224 are formed on the second insulation film. The electrontransport electrode 224 is formed such that it electrically contacts thesource electrode 212, while the first capacitor electrode 222 is formedsuch that it electrically contacts the ground line 214.

Organic BCB (benzocyclobutene), which is transparent material, isbeneficially used for the second insulation film 216. BCB is a materialhaving a dielectric constant less than 3, and a superior planarizingratio that results in a flat surface. An acryl or a polyamide can alsobe employed as the second insulation film 216.

Referring to FIG. 5e, a dielectric layer 226 is then formed over thesecond insulation film, over the first capacitor electrode 222, and overthe electron transport electrode 224. Then, a third contact hole 228 isformed through the dielectric 226 such that a portion of the electrontransport electrode 224 is exposed. Then, a pixel electrode 230(alternatively referred to as a second capacitor electrode 230) isformed on the dielectric layer 226. Moreover, the pixel electrode 230 isformed such that it has an electrical connection with the electrontransport electrode 224. As the pixel electrode acts as second capacitorelectrode, the storage capacitor “Cst” is completed. The storagecapacitor “Cst” is comprised of the first capacitor electrode 222, thepixel electrode 230, and the dielectric layer 226.

FIG. 5f illustrates a step of applying a light-sensitive material 232that is used as a converter that converts an external signal (X-rays)into electron-hole pairs. Beneficially, the light-sensitive material 232is an amorphous selenium compound that is deposited to a thickness of100 to 500 μm by an evaporator. Alternatively, an X-ray-sensitivematerial having low dark conductivity and a high sensitivity to externalsignals, for example HgI₂, PbO₂, CdTe, CdSe, Thallium bromide, orCadmium sulfide can also be used. When such a light-sensitive materialis exposed to X-rays, electron-hole pairs are produced in thelight-sensitive material in accordance with the strength of theirradiated light.

After the application of the light-sensitive material 232, a transparentconductive electrode 234 that transmits X-ray is formed over thelight-sensitive material 232.

When a voltage is applied to the conductive electrode 234 while X-raysare being irradiated, electron-hole pairs are formed in thelight-sensitive material. These pairs are separated into electrons andholes, one species of which is gathered to the pixel electrode 230 andstored as charge in the storage capacitor “Cst”.

When the gate electrode turns the TFT ON, the stored electric chargesare transferred to an external image display device (not shown) and areused to form an X-ray image.

After stopping the mentioned switching operation, the residual chargesare transferred to the ground line 214. Therefore, the ground line 214acts as the reset switch.

The present invention provides an X-ray image sensor having a simplicityof processing steps while forming ITO (indium tin oxide) electrodes.Moreover, the present invention provides a method of fabricating anX-ray image sensor that can have improved yields owing to a reduction inthe processing steps.

Furthermore, the invention provides a more reliable X-ray image sensorhaving fewer short-circuits between the TFT electrodes and capacitorelectrodes. Furthermore, the invention can reduce parasitic capacitancesthat affect the drain line. Therefore, noise can be decreased.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for fabricating an X-ray image sensor,comprising: forming a gate electrode on a substrate; forming a firstinsulation film over the substrate and the gate electrode; forming anamorphous silicon film on the first insulation film and over the gateelectrode; forming a doped amorphous silicon film on the amorphoussilicon film; forming source and drain electrodes on the doped amorphoussilicon film such that the source and drain electrodes are spaced apartfrom each other; forming a channel region on the amorphous silicon layerby eliminating the doped amorphous silicon film between the source anddrain electrodes; forming a ground line on the first insulation filmsuch that the ground line is spaced from the source and gate electrodes;forming a second insulation film over the ground line, the channelregion, the source and drain electrodes, and the first insulation film;forming first and second contact holes through the second insulationfilm such that a portion of the ground line is exposed by the firstcontact hole and a portion of the source electrode is exposed by thesecond contact hole; forming a first capacitor electrode on the secondinsulation film such that the first capacitor electrode is electricallyconnected to the ground line through the first contact hole; forming anelectron transport electrode on the second insulation film, the electrontransport electrode having an electrical connection with the sourceelectrode through the second contact hole; forming a dielectric layerover the second insulation film, the first capacitor electrode and theelectron transport electrode; forming a third contact hole through thedielectric layer; forming a second capacitor electrode on the dielectriclayer such that the second capacitor electrode has an electricalconnection with the electron transport electrode through the thirdcontact hole; forming a light-sensitive material on the pixel electrode;and forming an X-ray transparent conductive electrode on thelight-sensitive material.
 2. A method fabricating, an X-ray image sensoraccording to claim 1, wherein the first and second insulation layers areformed from a material selected from the group consisting of BCB(benzocyclobutene), acryl, and polyamide.
 3. The method for fabricatingan X-ray image sensor according to claim 1, wherein the first capacitorelectrode and the second capacitor electrode are formed from indium. 4.A method for fabricating an X-ray image sensor according to claim 3,wherein the first capacitor electrode and the second capacitor electrodeare formed from indium tin oxide.
 5. A method for fabricating an X-rayimage sensor according to claim 1, wherein the X-ray-sensitive materialis formed from a material selected from the group consisting, of HgI₂,PbO₂, CdTe, CdSe, Thallium Bromide, and Cadmium Sulfide.
 6. A method forfabricating an X-ray image sensor according to claim 1, wherein theX-ray-sensitive material is formed from amorphous selenium.
 7. A methodfor fabricating an X-ray image sensor according to claim 6, whereinamorphous selenium is at least 100 μm thick.
 8. A method forfabricating, an X-ray image sensor according to claim 7, whereinamorphous selenium is less than 500 μm thick.